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The
new JILA technique uses infrared laser light in many different
colors, or frequencies, to identify trace levels of different
molecules at the same time. For example, water molecules
(blue) and ammonia molecules (green) absorb light at very
specific characteristic frequencies. The pattern of frequencies
absorbed forms a "signature" for identifying
the molecules and their concentrations.
Credit: Jeffrey Fal, JILA
For
an animation, see www.nist.gov/public_affairs/images/frequency_comb_animation.htm.
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Boulder,
Colo. -- Physicists at JILA have designed and demonstrated
a highly sensitive new tool for real-time analysis of the
quantity, structure, and dynamics of a variety of atoms and
molecules simultaneously, even in miniscule gas samples. The
technology could provide unprecedented capabilities in many
settings, such as chemistry laboratories, environmental monitoring
stations, security sites screening for explosives or biochemical
weapons, and medical offices where patients’ breath
is analyzed to monitor disease.
Described
in the March 17 issue of Science,* the new technology
is an adaptation of a conventional technique, cavity ring-down
spectroscopy, for identifying chemicals based on their interactions
with light. The JILA system uses an ultrafast laser-based
“optical frequency comb” as both the light source
and as a ruler for precisely measuring the many different
colors of light after the interactions. The technology offers
a novel combination of a broad range of frequencies (or bandwidth),
high sensitivity, precision, and speed. A provisional patent
application has been filed.
JILA
is a joint institute of the National Institute of Standards
and Technology (NIST), a non-regulatory agency of the U.S.
Department of Commerce, and the University of Colorado at
Boulder.
“What
a frequency comb can do beautifully is offer a powerful combination
of broad spectral range and fine resolution,” says NIST
Fellow Jun Ye, who led the work described in the paper. “The
amount of information gathered with this approach was previously
unimaginable. It’s like being able to see every single
tree of an entire forest. This is something that could have
tremendous industrial and commercial value.”
Frequency
combs are an emerging technology designed and used at JILA,
NIST, and other laboratories for frequency metrology and optical
atomic clocks, and are being demonstrated in additional applications.
NIST/JILA physicist John (Jan) Hall shared the 2005 Nobel
Prize in physics in part for his contributions to the development
of frequency combs [http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm].
In the application described in Science, the frequency
comb is used to precisely measure and identify the light absorption
signatures of many different atoms and molecules.
The JILA
system described in Science offers exceptional performance
for all four of the primary characteristics desired in a cutting-edge
spectroscopic system:
- The
system currently spans 125,000 frequency components of light,
or 100 nanometers (750-850 nm) in the visible and near-infrared
wavelength range, enabling scientists to observe all the
energy levels of a variety of different atoms and molecules
simultaneously.
- High
resolution or precision allows scientists to separate and
identify signals that are very brief or close together,
such as individual rotations out of hundreds of thousands
in a water molecule. The resolution can be tweaked to reach
below the limit set by the thermal motion of gaseous atoms
or molecules at room temperature.
- High
sensitivity—currently 1 molecule out of 100 million—enables
the detection of trace amounts of chemicals or weak signals.
With additional work, the JILA team foresees building a
portable tool providing detection capability at the 1 part
per billion level. Such a device might be used, for example,
to analyze a patient’s breath to monitor diseases
such as renal failure and cystic fibrosis.
- A
fast data-acquisition time of about 1 millisecond per 15
nm of bandwidth enables scientists to observe what happens
under changing environmental conditions, and to study molecular
vibrations, chemical reactions, and other dynamics.
By comparison,
conventional cavity ring-down spectroscopy offers comparable
sensitivity but a narrow bandwidth of about 1 nanometer. A
more sensitive “optical nose” technique developed
at NIST can identify one molecule among 1 trillion others,
but can analyze only one frequency of light at a time. Other
methods, such as Fourier transform infrared spectroscopy,
provide large bandwidths and high speed but are not sensitive
enough to detect trace gases.
The research
at JILA is supported by the Air Force Office of Scientific
Research, NIST, Office of Naval Research, National Aeronautics
and Space Administration, and National Science Foundation.
As a
non-regulatory agency of the Commerce Department’s Technology
Administration, NIST promotes U.S. innovation and industrial
competitiveness by advancing measurement science, standards
and technology in ways that enhance economic security and
improve our quality of life.
* M.J.
Thorpe, K.D. Moll, R.J. Jones, B. Safdi, and J. Ye. 2006.
Broadband cavity ringdown spectroscopy for sensitive and rapid
molecular detection. Science. March 17.
Background:
Using a Frequency Comb to Enhance Spectroscopy
Cavity ring-down
spectroscopy identifies atoms or molecules by the way they
absorb laser light as it is repeatedly reflected and dissipates
inside a mirrored vacuum cavity.
The JILA system
uses a laser that emits a broad range of colors. The laser
generates about 380 million pulses per second, each lasting
about 20 femtoseconds (quadrillionths of a second). The laser
light is tuned to the “resonant frequency” of
the cavity, such that all of the many different wavelengths
of light—all “harmonics” of a single basic
wave size—fit perfectly between two special mirrors.
The distance between the mirrors is adjusted using tiny motors
to select the resonant frequency of the cavity. The mirrors
inside the laser are then rotated to match the laser frequencies
to those of the cavity.
The light is repeatedly
reflected inside the cavity until the laser is turned off,
after which all of the energy is gradually lost in a few microseconds.
If atoms or molecules are placed inside the cavity, they absorb
some of the light energy at frequencies where they switch
energy levels, vibrate, or rotate, and the light dissipates
faster at those frequencies.
A beam of “white
light” is emitted from the cavity during the dissipation
process and separated into a rainbow of colors, which are
detected in sets of color bands. Computer software can analyze
the change in the decay time of selected channels of different
frequencies simultaneously. The results are rapidly matched
against a catalog of absorption signatures of known atoms
and molecules.
The JILA method
was demonstrated by conducting a variety of experiments with
argon atoms and acetylene, water, oxygen, and ammonia molecules.
The scientists demonstrated real-time, quantitative measurements
of traces of gas, the frequencies and strength of signals
signifying changes in energy levels, and other changes due
to collisions and temperature changes inside the cavity.
For instance, the
system identified a change in the acetylene signal, detected
as a faster dissipation time, as the pressure of the background
argon gas was increased and collisions between the gases increased.
The signal resolution was sufficient to reveal spectral information
that is difficult to access because it is below the physical
limits set by the thermal motion of the gas molecules. In
addition, analyses of water, ammonia, and oxygen demonstrated
that nearly the entire 100 nm spectral range can be probed
simultaneously. This combination of high resolution and broad
bandwidth is unprecedented.
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